An excessive driving force when the vehicle is traveling on a slippery road such as a snow-laden road tends to cause a slippage of the driving wheels. This results in a reduction of gripping force of the tires, acceleration characteristics and drivability. To deal with such a situation, a driving force control apparatus or a so-called traction control apparatus has been developed and has been practically used.
In the traction control apparatus, a slip of the driving wheels is initially detected. When the slip is determined to be considerable, the output of the engine is forcibly and rapidly suppressed to reduce the driving force of the driving wheels independent of the depression amount of the acceleration pedal by the driver. This suppresses slippage of driving wheels to improve the starting and acceleration characteristics of the vehicle on a slippery road such as a snow-laden road.
A control device for reducing the engine output to suppress a slip includes the following:
(1) Throttle control (throttle valve of the intake system is closed)
(2) Ignition timing retard control (ignition timing of the ignition plug is retarded)
(3) Fuel control (fuel cutting or fuel injection amount control)
(4) Cylinder number control (operation of a predetermined number, among a plurality of cylinders, is suspended).
(5) Brake control (brake is operated to suppress the slip).
Most of the known traction control apparatuses use a combination of the throttle control and the ignition timing retard control. According to the throttle control, the engine output can be controlled smoothly and over a wide range. In an abrupt start or when the road surface condition suddenly changes from a dry road to a frozen road, a sharp slip occurs. The throttle control is not able to control such a sharp slip. When a sharp and excessive slip occurs, the ignition timing retard control is temporarily operated to suppress a sharp slip with a good response.
Initially, the traction control apparatus was applied to a 2-wheel driving (2-WD) vehicle. However, even a 4-wheel driving (4-WD) vehicle, with increased engine output and relegated to use with spiked tires, may undergo a slip on a slippery road. Accordingly, development has been conducted to apply the traction control apparatus to the 4-wheel driving vehicle.
The development has been directed to the following vehicle and apparatus:
(a) a 4-wheel driving vehicle provided with a limited slip differential (LSD) at the rear wheels (brief construction thereof will be described later);
(b) the traction control apparatus (driving force control apparatus) to be equipped as a mechanism using a combination of the throttle control and the ignition timing retard control (brief operation thereof will be described later).
Here, the 4-WD vehicle (a) provided with a rear wheel LSD will be briefly described. In the LSD, the differential gear itself is of a conventional bevel gear type. The LSD includes a viscous coupling mounted between the left differential side gear and the differential case. The right and left drive shafts are connected by the viscous coupling.
No differential effect appears when the rotational speeds of the right and left drive shafts are same, and the torque is equally distributed. When the vehicle runs on a split road (frictional coefficients of the road surface contacting the driving wheels differ between the right and left sides) or the like so that only one wheel makes a slip and its rotational speed is increased, the torque allocation to the opposite wheel is increased. That is, the differential action is limited when one wheel slips. Such a slip can occur on a rough road such as a split road or a snow-laden road or during the pulling out of a mire, for example.
FIG. 5 shows the power transmission system of a 4-wheel driving vehicle provided with a rear wheel LSD. As shown in FIG. 5, the torque output from the engine is transmitted to a center differential 401 through a torque converter and a transmission. The center differential 401 has a function to distribute the torque to front wheels 402 and rear wheels 403 in a predetermined ratio. The torque is then transmitted from the center differential 401 to the front wheels 402 through a front wheel output shaft 404, a front differential 405, and a front wheel shaft 406. Further, the torque is transmitted from the center differential 401 to the rear wheels 403 through a rear wheel output shaft 407, a propeller shaft 408, an LSD 409, and a rear wheel shaft 410. The LSD 409 incorporates a differential limit mechanism 409a.
Next, operation of the traction control apparatus (b) will be briefly described. In the traction control apparatus for use on a 4-wheel driving vehicle, a detailed construction of which will also be described in the embodiments of the present invention, when a slip occurs the throttle valve is forcibly closed. Further, when an abrupt slip occurs, a retard command L is output from the computer to effect ignition timing retard control temporarily.
In the known ignition timing retard control, as shown in FIG. 6, a correction target driving wheels speed V.sub.OTC (according to the vehicle body speed, which will also be described in the embodiments of the present invention) is subtracted from a left front wheel speed V.sub.FL, right front wheel speed V.sub.FR, left rear wheel speed V.sub.RL, and right rear wheel speed V.sub.RR to determine slip amounts DVS.sub.1, DVS.sub.2, DVS.sub.3, and DVS.sub.4.
A slip select averaging unit 601 selects the greatest and the next greatest of the slip amounts DVS.sub.1, DVS.sub.2, DVS.sub.3, and DVS.sub.4, averages the two selected values, and outputs the averaged slip amount as an average slip amount DVS.sub.a. A differentiation unit 602 differentiates the average slip amount DVS.sub.a to determine a slip rate GDVS.sub.a. A retard control set/reset determination unit 603 outputs a retard command L when the average slip amount DVS.sub.a is greater than a predetermined value and the slip rate GDVS.sub.a is greater than a predetermined value. The ignition timing retard control is carried out by the output of the retard command L to decrease the driving force of the driving wheels, thereby suppressing an abrupt slip.
In the known system as shown in FIG. 6, the two largest values of the slip amounts are averaged to determine the slip amount DVS.sub.a, which is differentiated to determine the slip rate GDVS.sub.a. Therefore, when an abrupt slip occurs at only one of the four wheels, the slip rate GDVS.sub.a is also increased, and the retard control is executed.
In the 4-wheel driving vehicle having the rear LSD, when an abrupt slip occurs at only a single wheel, the vehicle body can create a stable running condition by the function of the rear LSD. Thus, it is better not to decrease the driving force of the driving wheels. However, when the known traction control apparatus is used, the driving force is decreased even though an abrupt slip occurs at only one wheel. Thus, the function of the LSD has not been effectively obtained. That is, with the LSD, even under a condition where the driving force need not be decreased, when the vehicle runs on a split road or the like, the retard control is frequently made to decrease the driving force. This deteriorates the drive feeling.
Particularly in the retard control, since the torque is immediately decreased by the retard, a shock due to torque variation tends to occur. Thus, if the retard control occurs excessively, drive feeling is deteriorated.
Further, when similar control is made by braking the wheels or by cutting fuel to decrease the fuel supply amount, a shock tends to occur due to an abrupt change of torque, resulting in a similar problem.
The technology having the above-described problems is also disclosed in Japanese Laid-open Patent Publication 2-140439.
In the technology described in Japanese Laid-open Patent Publication 2-140439, in a 4WD vehicle, a greatest wheel speed and a least wheel speed are selected from the individual wheel speeds. Then, a difference in speed between the greatest and least values, or a speed ratio of the greatest and least wheel speeds, is determined. Occurrence of a slip is determined when the speed difference or speed ratio is greater than a preset value to control the opening of the throttle valve. By making such a determination, a problem of excessive reduction in the driving force occurs since output control of the engine is effected when only one of the four wheels slips. In particular, when the determination method is applied to a vehicle in which engine output control is carried out by a high-response method such as ignition timing control or fuel cutting, or the driving force is controlled by braking control, not only the acceleration is reduced, but also driving comfort is impaired due to vibrations caused by repetitions of control.